Cosmic rays can produce backgrounds for the experiment in a number of ways. Although \(\sim 10^{14}\)  cosmic ray muons will pass through the detector over its lifetime, their trajectory is downward going, and the timing resolution is sufficient to prohibit mis-reconstruction in the opposite direction, let alone two tracks to form a vertex. Nevertheless, upwards going tracks can still be produced primarily through back-scattering off of the rock beneath the detector as well as the sides. For single tracks, these backgrounds are not substantial: the vast majority do not have appreciable range and are suppressible with the same techniques that IP muons are. A more pernicious background is the production of strangeness-bearing SM LLPs, such as \(K_{\rm L}\)  and \(\Lambda \) . The MATHUSLA collaboration is studying the rates for these processes from cosmic muons and protons in Geant4 [1]} simulations using the energy and angular distribution computed by Parma [2]}, [3]}, [4]}. Approximately \(\mathcal {O}(10^7)\)  such particles will be produced in the rock beneath the detector but re-emerge inside the detector before decaying. Nevertheless, the vast majority will not be reconstructable by the detector, as they are preferentially produced with large boost in directions parallel to ground, while those produced pointing towards the tracking layers are disproportionately soft. Several handles are available to suppress even this background: neutral particle production is correlated with nearby activity in the floor, and topological consistency with production at the IP is also a significant constraint.
